As a minimum requirement for satisfactory combustion of fuel in a gas turbine engine, it is essential for the fuel to be atomized into a fine spray of small droplets which are evenly circumferentially distributed at all operating conditions. This necessity has required the development of complex and sophisticated fuel nozzles. During this development, it has become common practice to use a swirl-atomizer in which the fuel is supplied at high pressures to a swirl chamber in which a free vortex is formed. Consequently, the fuel issues from the discharge orifice of the swirl chamber as a thin sheet of conical section which breaks up into a spray of drops by its high velocity interacting with the surrounding air. These nozzles are known typically as pressure atomizers. It has also become common to combine two pressure atomizers, one of low flow capacity known as the "primary" and the other of high flow capacity, known as the "secondary" into a single fuel nozzle. This type of nozzle has conventionally become known as a dual orifice nozzle, as shown in U.S. Pat. No. 3,013,732, the entirety of which is incorporated herein by reference.
To obtain improved fuel atomization over the pressure atomizer, it has become common practice to use high velocity and/or high pressure air as a means of atomizing the fuel. When the air is supplied from a source external to the engine, the nozzle is known as an air-assisted type. When the air is available from inside the engine, it is known as an airblast nozzle.
There are many applications where it is deemed necessary or desirable to combine an air-atomizing nozzle with a pressure atomizing nozzle, such as shown in U.S. Pat. No. 3,912,164, the entirety of which is incorporated herein by reference. In such an arrangement, the pressure atomizer is used for the low fuel flow rate conditions, such as starting the engines, while the air atomizer is used for the higher fuel flow rates. This combination has become conventionally known as a hybrid nozzle.
In some types of airblast nozzles, it may be difficult to obtain optimum spray characteristics due to limitations regarding nozzle shroud and swirl vane geometry. These limitations might arise due to restrictions inside the engine or due to requirements relating to overall geometry. In such a nozzle the fuel, before it has become evenly distributed into a thin sheet, may enter the air stream in a location of high air turbulence (such as in the wake of a swirl vane). This may cause incomplete fuel atomization which will result in less than optimum engine performance.